Radial tire for heavy load vehicles

ABSTRACT

A radial tire for heavy load vehicles comprising a tread portion having an outer contour composed of a relatively transversely flat main region and a conical region located at both end edges of the main region. A number of thin incisions extend across the conical region having inner ends not reaching to the inside of the main region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a radial tire for heavy load vehicles such astrucks, buses or the like, which is constructed to effectively suppressa wandering phenomenon of the tire.

2. Description of the Prior Art

In the specification, the wandering phenomenon shall be understood tomean a phenomenon experienced often by a driver of a heavy load vehiclesuch as a large type truck or the like when it runs in a wheel trackformed when a highly used paved road surface becomes depressed or worndue to the vehicle weight, particularly when a driver tends to changethe direction of the vehicle or to get away the vehicle from the wheeltrack in order to avoid an obstacle scattered thereon.

The radial tire for trucks and buses has a boundary between a tread anda sidewall provided with a sharp edged square shoulder has particularlybeen subjected to the wandering phenomenon. As a result, it hasheretofore been proposed to use a round edged round shoulder tire forthe purpose of alleviating the resistance to shocks subjected to theshoulder edge when the tire rides across the wheel track and hence ofsuppressing the wandering phenomenon.

Such a round shoulder configuration is affected by the bending of thetire produced by the dynamic load due to the amount of load or thevibration when the vehicle not only runs around the corner but also runsstraight ahead. As a result, the round shoulder functions as a partialground contact region. That is, this partial ground contact region is arounded portion and hence is subjected to a ground contact pressurewhich is lower than that subjected to a relatively flat tread. The roundshoulder therefore tends to easily respond to the local step or movementwhich is liable to be occur between the round shoulder and the groundsurface in the ground contact region, thereby inducing an abnormalpremature wear, that is, a shoulder wear.

SUMMARY OF THE INVENTION

An object of the invention, therefore, is to provide a radial tire whichcan effectively suppress the wandering phenomenon without inducing theshoulder wear.

A feature of the invention is the provision in a radial tire for heavyload vehicles constructed to effectively suppress the wanderingphenomenon comprising a pair of annular bead bundles, a radial carcasstoroidally extending across said bead bundles, and a tread portionincluding a reinforcing belt superimposed about said radial carcass. Thetread portion has an outer contour composed of a relatively transverselyflat main region exclusively adapted to make contact with ground whenthe tire normally runs and a conical region located at both end edges ofthe main region and inclined at a given angle with respect to a tangentdrawn from the tread in parallel with the rotary axis of the tire. Theimprovement comprises a number of thin incisions extending across theconical region and circumferentially distant apart from each other, theinner end of the thin incision being located on at least the outersurface of the tread at a position which is not reached to the inside ofsaid main region.

The term normal running shall be understood to mean substantially thatrectilinear running condition of a vehicle which includes a change-overof the tracks one from the other on a good ground surface not providedwith a plurality of wheel tracks, but excludes a riding over of thewheel tracks formed on a bad ground surface.

Further objects and features of the invention will be fully understoodfrom the following detailed description with reference to theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial development view of a tread of a radial tireaccording to the invention; and

FIG. 2 is a cross-sectional view of a left half contour of the treadshown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a development of a tread portion of a radial tire for heavyload vehicles according to the invention and FIG. 2 shows a contour of aleft half cross-section of the tread portion shown in FIG. 1.

The radial tire for heavy load vehicles according to the inventioncomprises a pair of annular bead bundles, a radial carcass toroidallyextending across these bead bundles and a tread portion superimposedabout this radial carcass and including a reinforcing belt formed ofinextensible material, all of these members being not shown.

It is a matter of course that the tread portion shown in FIG. 1circumferentially extends along the tire periphery in an endless manner.

In the drawings, reference letter T designates a tread portion composedof a main region T₁ and a conical region T₂ formed at both ends of themain region T₁.

In the main region T₁ of the tread portion T are arranged two pairs ofmain grooves 1 symmetrically arranged with respect to the equatorialplane C-C of the tire.

The conical region T₂ formed at both ends of the main region T₁ of thetread portion T has a truncated conical surface defined by a line L₂inclined at an angle θ of 30° to 60° with respect to a tangent L drawnfrom the tread T in parallel with the rotary axis of the tire.

That is, the conical surfaces L₂, L₂ make a vertical angle of 60° to120° with respect to the rotary axis of the tire.

In the conical regions T₂ a number of thin incisions S are arrangedcircumferentially distant apart from each other by a given distance P.

The reasons why the angle θ is limited to the above mentioned rangebetween 30° and 60° are as follows. If the angle θ is smaller than thelower limit of 30°, there is a risk of shoulder wear being induced in arange from the conical region T₂ to the main region T₁ in dependencewith the condition of the tire in use. If the angle θ is larger than theupper limit of 60°, the conical region T₂ becomes difficult to matchwith ridges projected from the both sides of the wheel track, therebyreducing the effect of improving the wandering phenomenon.

The use of the conical region T₂ ensures a further improvement in thewandering phenomenon if compared with the conventional round shouldertire and provides the important advantage that the abnormal wear of theconical region T₂ to be induced in the case of the tire rectilinearrunning can be prevented.

It is desirous to make a percentage ratio of a width W₁ of the conicalregion T₂ to a width W_(t) of the main region T₁ a value which lieswithin a range between 2% and 7% for the purpose of effectivelysuppressing the wandering phenomenon and of maintaining the width W_(t)necessary for bringing the tread into contact with ground when the tireruns normally.

Experimental tests have demonstrated the result that the use of theconical region T₂ only slightly induces the abnormal wear as in the caseof the round shoulder tire in the case of cornering and that thisabnormal wear can effectively be prevented by making the rigidity of theconical region T₂ small. For this purpose, provision may be made of anumber of thin incisions S.

It is convenient to form such numerous thin incisions S in the conicalregion T₂ when the tire is shaped in a vulcanization mold. The use of anincision S having a gap which lies within a range between 0.3 mm and 1mm ensures a convenient molding of the incision S without deterioratingits object.

A depth d of the thin incision S, a distance P between adjacentincisions S, in the circumferential direction of the tire and directionthereof are particularly important with respect to their influence uponthe rigidity of the conical region T₂. In addition, the inner endposition e of the incision S on the tread outer surface is alsoimportant with respect to its influence upon the uneven wear due to theincision S.

Inventors' experimental tests have demonstrated that the base of theincision S substantially reaches to an extrusion line L₃ passing throughthe bases of the main grooves 1 of the tread T and that the incision hasa depth d which is preferably at most 1.3 times larger than the depth Hof the main groove 1. This causes the thin incision S to maintain itseffect even when the tread has completely been worn.

It is effective to arrange the thin incisions S in such a direction thatthey substantially extend across the conical region T₂ and arecircumferentially distant apart from each other by a distance which iswithin a range between 1% and 5% of the width W_(t) of the main regionT₁ of the tread portion T. If the distance P is smaller than 1% of thewidth W_(t) of the main region T₁, the manufacture of the tire becomesdifficult and the rubber between the incisions becomes broken. If thedistance P is larger than 5% of the width W_(t) of the main region T₁,the effect of decreasing the rigidity of the conical region T₂ becomesin vain.

If the surface of the thin incisions as a whole is inclined at an anglewhich is larger than 45° with respect to the circumferential directionof the tire, the effect of decreasing the rigidity of the conical regionT₂ could not be obtained and abnormal wear occurs at both edges e of themain region T₁. It is particularly important that the inner end of thethin incision S should not inwardly exceed both edges e on the treadouter surface of the main region T₁ and should not reach to therelatively flat surface of the main region T₁. If the thin incision S isarranged such that its inner end inwardly exceeds the above mentionededge e, heel and toe wear is produced at a position near both edges ofthe main region T₁ of the tread portion T, thereby forming steps infront and rear of the thin incision S.

Further experimental tests have yielded the result that the inner end ofthe thin incision S should not inwardly exceed a perpendicular line L₄drawn from the edge e of the main region T₁ toward the rotary axis ofthe tire on the outer surface of the tread portion T and substantiallyinside the rubber and that the outer end of the thin incision S shouldnot outwardly exceed a perpendicular line L₅ drawn from the edge M ofthe conical region T₂ toward the rotary axis of the tire.

The depth H of the main groove 1 and the depth d of the thin incision Sshall be understood to be measured on the outer surface of the treadportion T in a direction normal thereto.

A radial tire having a size of 10.00R20 14PR was manufactured for trialand its wandering ability and presence and absence of shoulder wear weretested. The tire was composed of a carcass including one radiallyarranged steel cord ply and a belt superimposed about the carcass andincluding four steel cord plies. The tread portion T is provided withfour main grooves 1 each having a depth H of 14 mm.

The conical region T₂ has dimensions shown in the following Table.

    ______________________________________                                        Conical  Width (W.sub.1 /W.sub.2 × 100)                                                           5.3                                                 region T.sub.2                                                                         Angle θ (°)                                                                       45                                                           Gap (mm)         0.5                                                          Distance (P/W.sub.t × 100)                                                               2.6                                                 Incision S                                                                             Depth (mm)       17 mm                                                                         90° with respect to                                   Direction        the circumferential                                                           direction of the tire                               ______________________________________                                    

The tire constructed as above described was mounted on a rim of 7.50 Vand inflated under an internal pressure of 7.25 kg/cm². Then, the tirewas mounted on a truck and the load was adjusted so as to make the wheelweight 2,425 kg. A running test of 100,000 km of this truck was carriedout.

The wandering ability was tested during the above mentioned runningtest. An adverse torque exerted to the handle when the radial tireaccording to the invention runs in the wheel track and when the tirerides across the ridge of the wheel track and then is returned againinto the wheel track and a frequency of the self-restoring torquesubjected to the handle or the like were significantly improved ifcompared with those of the conventional radial tire not provided withthe conical region T₂ including the thin incision S.

In addition, the use of the thin incision S in the conical region T₂ensures substantially no occurrence of the shoulder wear during the100,000 km running of the tire and hence results in a complete wear ofthe tire.

What is claimed is:
 1. In a radial tire for heavy load vehiclesconstructed to effectively suppress wandering phenomenon having a pairof annular bead bundles, a radial carcass toroidally extending acrosssaid bead bundles, and a tread portion including a reinforcing beltsuperimposed about said radial carcass, said tread portion having anouter contour composed of a relatively transversely flat main regionexclusively adapted to make contact with ground when the tire runsnormally and a conical region located at both end edges of said mainregion and inclined at an given angle of 30°-60° with respect to atangent drawn from the tread in parallel with the rotary axis of thetire, the improvement comprising; a number of thin incisions extendingacross said conical region and having inner ends each located on atleast the outer surface of the tread at a portion not reaching theinside of said main region, said incisions being circumferentiallyspaced apart from each other by a distance which is within a rangebetween 1% and 5% of the width of the main region of the tread portion,said conical region having a width measured in a direction parallel tothe rotary axis of the tire and lying within a range between 2% and 7%of the width of said main region and said incisions having a gap withina range between 0.3 mm and 1 mm.
 2. The radial tire according to claim1, wherein said thin incision has a depth in the radial direction of thetire and measured on the extension line drawn from the base of the maingroove of the tread which lies within a range between 1 and 1.3 timeslarger than the depth of the main groove.
 3. The radial tire accordingthe claim 1, wherein said thin incision is located between two verticallines drawn from the inner and outer ends of said conical region towardthe rotary axis of the tire.